Among various liquid crystal panels, an active-matrix liquid crystal panel is formed, for example, by sequentially and selectively forming a semiconductor layer, an insulating layer, and a conductive layer on a large substrate, such as a glass substrate, to form a plurality of panel areas provided with active elements, passive elements, electrodes, and other components, and by cutting these panel areas from the large substrate. This active-matrix substrate is used for an electro-optical device. Specifically, it is used in an electro-optical device in which a liquid crystal is sandwiched by the active-matrix substrate and an opposing substrate. In the active-matrix substrate, a number of pixels are formed in a matrix and they form a pixel section. In the pixel section, thin-film transistors (hereinafter called TFT: thin film transistor) are formed and a voltage is applied to pixel electrodes through the TFTs.
In such an active matrix substrate, when poly-silicon (Poly-Si) is used as a semiconductor material to form the TFTs, since transistors and other devices constituting peripheral circuits such as a shift register and a driving circuit can be formed in the same process, high integration is enabled.
In such an active matrix substrate, when poly-silicon TFTs are formed as transistors, since the active-matrix substrate can be formed in a low-temperature process, it is advantageous that a glass substrate made from silica glass or non-alkaline glass can be used as an insulating substrate.
Since the glass substrate is likely to be charged, however, when static electricity is discharged from the charged substrate, TFTs and other devices working as active elements may be destroyed (hereinafter called electrostatic destruction) by static electricity.
In an active-matrix substrate, an alignment layer is formed on the glass substrate on which active elements, passive elements, and electrodes are formed, to align liquid-crystal molecules in a prescribed direction. In a rubbing process for the alignment layer, however, the substrate is charged with high-voltage static electricity generated by the rubbing. When static electricity is discharged from the charged substrate, the TFTs and other devices working as active elements may be electrostatically destroyed.
More specifically, an organic high-polymer film made from polyimide resin or the like is formed on the glass substrate on which the active elements and other devices are formed, and the rubbing process is applied to a surface of this resin film to align the liquid-crystal molecules, in which the surface is rubbed with textile fabrics made from fiber, such as rayon and nylon, in a constant direction at a prescribed load. In this process, friction between the resin film and the fiber generates high-voltage static electricity. This static electricity charges the substrate itself, or is discharged over insulation to electrostatically destroy semiconductor devices, such as the TFTs formed on the substrate.
According to knowledge which the inventors of the present application obtained, since poly-silicon TFTs and other devices formed in the low-temperature process at a maximum process temperature of about 400 to 600° C. have an extremely low dielectric strength, they are likely to be electrostatically destroyed. In some cases, they may cause a serious problem almost identical to a fatal error, in which the entire driving circuit does not function.
Accordingly, an object of the present invention is to provide an active-matrix substrate, an electro-optical device, and a method for manufacturing an active-matrix substrate, which provide a structure that can effectively prevent TFTs and other devices formed on a substrate from being destroyed by static electricity generated for some reason or by a rubbing process for a liquid-crystal alignment layer.